The Unit's continuing study of the vocal behavior of nonhuman primates this year focused on early vocal interaction and development, and the use of vocal interaction is social groups of adults. We have observed that squirrel monkeys use "chuck" calls in what appears to be a "conversational" manner. The acoustic structure of chucks exchanged by female squirrel monkeys varies with the chuck's position in a sequence, much like the voice changes which, in human languages, alter the meaning of a word (e.g., Yes? Yes!) to indicate a question or an answer. Using a technique where calls originating at different points in a chuck exchange are recorded and played back to animals under identical circumstances, we have established that squirrel monkeys can distinguish the original vocal context of chuck calls, but only if the calls are from members of their own social group. Animals were unable to differentiate the original context of calls recorded from strangers of the same species. Thus, at least some of the information in the chuck call is only available (or only used) for familiar speakers whose calls are well-known to the listener, effectively excluding outsiders from conversations. We continue to gather data on the vocal response of squirrel monkeys to the recorded chuck calls of relatives with whom they have been out of contact for years. The ability to recognize even calls they have never heard before suggests that "familiarity" may encompass some as yet unidentified common properties in the chuck calls of animals sharing genes or rearing environment. In our studies of early development, new data expand our findings on the importance of caregiving behaviors by individuals other then the mother. We have now documented the first evidence of a link between eye contact and vocal exchange between infants and caregivers in a monkey species. As in human infants, eye contact facilitates vocal responsiveness in the squirrel monkey infant. Much of the earliest social interaction is between all mothers, or aunts, and infants, probably because the infant is carried on the mother's back and cannot easily make eye contact with her. We continue to add subjects to our study of vocal production and usage in infant squirrel monkeys, and we are now beginning to see a developmental continum in the production of chucks, which do not mature until about 12 months of age or later. The evident lability of this call makes it a candidate for investigations of social influences on development.